The subscapular artery has two main branches, the circumflex scapular artery and thoracodorsal artery . The thoracodorsal artery supplies the latissimus dorsi muscle. The circumflex scapular artery passes through the triangular space and participates in the collateral circulation of the scapula ( see Laboratory Three). The posterior circumflex humeral artery passes with the axillary nerve through the quadrangular space to anastomos with the anterior circumflex humeral artery. Verify the course and relationships of these vessels by passing a probe through the quadrangular and triangular spaces. These arteries supply the surrounding muscles, humerus, and shoulder joint.